This invention relates to a system for towing a tow vehicle underwater, and more particularly to a system for maintaining the tow vehicle at a fairly stable predetermined distance above the sea floor.
The increasing demand for petroleum and petroleum products has given impetus to undersea research and exploration, with the object of such exploration being the location of petroleum deposits. Some of the principal tools in carrying out such undersea exploration are underwater sonar or acoustic seabed survey systems, combined sonar and TV systems, and the like. Such systems are used for obtaining information relating to horizontal seabed topography and sub-bottom structure or bathymetric profile as well as actual pictures of the sea floor. This information is useful, not only in assisting in the location of offshore oil deposits, but also in providing a better understanding of the oceans generally, their resources, and the geology of the earth. The closer to the sea floor the system can be placed, the more accurate and detailed is the information obtained.
Information about seabed topography and sub-bottom structure is typically gathered by towing behind a ship a so-called tow vehicle containing sonar transducers and other instruments. The transducers transmit sonar or acoustical signals toward the sea floor and receive reflections from the floor and from sub-floor layers. This information is then transmitted to recording devices on the ship where records of the information, for example in the form of line traces on chart paper, are made. In gathering such information, it is important that the tow vehicle be maintained at a fairly constant elevation close to the sea floor and that the yaw, heave and roll of the tow vehicle be minimized. This becomes especially difficult as the depth of the water increases since maintenance of the stability of the tow vehicle is carried out, to the extent possible, by controlling the speed of the ship and by operation of the winch (letting out or reeling in the cable). As cable length increases, such control becomes very difficult. In particular, the tow vehicle sinks in the water under its own weight and the weight of the cable, and then the depth of the vehicle is controlled by controlling the speed of the ship and the tow cable winch. If the cable length were very great, which would be the case of deep water operation, there would be a significant delay time between discovering that the tow vehicle should be raised (for example because an obstacle is being approached) or lowered (for example because a valley is being approached), and the actual raising or lowering of the tow body by increasing the ship speed (or reeling in the cable) or reducing the ship speed (or paying out the cable) respectively. Also, if the cable length is very great, very small speed changes in the ship cause the tow vehicle to change depth significantly thereby making it difficult (and risky to the tow vehicle) to maintain a stable elevation of the tow vehicle above the sea floor.
Because the tow vehicle is connected directly via the tow cable to the ship, erratic moves of the ship or cable are transmitted directly to the tow vehicle, possibly causing it to yaw, heave or roll. Also, since controlling the ships speed through the water is the method of maintaining the tow vehicle above the sea floor, any sudden stops of the ship may result in the tow vehicle "crashing" into the sea floor.